The invention relates generally to wall-flow honeycomb filters and extrusion dies for making the same. More specifically, the invention relates to a wall-flow honeycomb filter having a hexagonal channel symmetry and an extrusion die for making the same.
Wall-flow honeycomb filters are used in diesel exhaust systems to remove soot and ash from diesel exhaust. The conventional wall-flow honeycomb filter consists of a ceramic monolith body having longitudinal, parallel channels defined by porous walls. The channels are alternately end-plugged to form a checkered pattern of plugs at the end faces of the monolith body. The channels having their ends plugged at the inlet end face of the monolith body may be referred to as outlet channels, and the channels having their ends plugged at an outlet end face of the monolith body may be referred to as inlet channels. The cross-section of the inlet and outlet channels is typically square because square cells are easier to manufacture and lend themselves to a regular pattern of alternating inlet and outlet channels having equal cross-sectional areas for low pressure drop.
Diesel exhaust enters the wall-flow honeycomb filter through the inlet channels, flows through the porous walls into the outlet channels, and exits through the outlet channels, with the porous walls retaining a portion of the soot and ash in the exhaust. As soot and ash accumulate on the porous walls, the effective flow area of the inlet channels decreases. The decreased effective flow area creates a pressure drop across the honeycomb filter, which exerts a back pressure against the diesel engine. To maintain the back pressure exerted against the diesel engine at an acceptable limit, thermal regeneration is used to remove the soot trapped in the honeycomb filter. During thermal regeneration, the filter can experience high thermal gradients, which can lead to higher thermal stresses that can crack the filter. It is thus desirable that the honeycomb filter has a cell structure that is resistant to cracking during thermal regeneration.
The honeycomb filter is typically wrapped in a mat and inserted in a metal can prior to use in an exhaust system. When the honeycomb filter is inserted in a can, the forces required to restrain the honeycomb filter within the can are uniformly distributed along the periphery of the monolith body, perpendicular to the skin of the monolith body. These forces have the greatest impact for the honeycomb filter with square cells when applied at 45° positions to the square cells, that is, in a direction along the diagonals of the square cells. When loaded at this angle, the walls defining the square cells cannot function as columns under compression, and the honeycomb filter is less rigid. In this state, the walls are subjected to high deflections, which generate bending moments and undesirable tensile stresses in the honeycomb filter.
From the foregoing, it would be an advancement in the art to have a wall-flow honeycomb filter having a more compliant cell structure than the conventional square cell structure. Desirably, the honeycomb filter having the more compliant cell structure would be able to maintain a low pressure drop during use in an exhaust system.